There is demand for attaching, to a small-sized motor, a device for detecting the rotational speed and position of the motor. Known detecting devices of this kind perform magnetic detection by use of a magnet and a Hall element, mechanically turn on/off electrical continuity between two brushes, or perform optical detection by use of a photodiode (light-emitting element) and a phototransistor (light-receiving element). The present invention relates to this optical encoder device. A small-sized motor having such an optical encoder device can be used in OA apparatus which requires rotational control, such as a printer.
FIG. 18 shows a first prior art technique for attaching an optical encoder to a motor (refer to Patent Document 1). FIG. 18(A) shows a small-sized motor in which a code wheel is attached to a motor shaft; (B) shows a base member on which a photosensor module is mounted; and (C) shows a state in which the small-sized motor and the photosensor module are assembled together. In the assembled state shown in (C), the code wheel having an optical modulation track is disposed in a gap of the photosensor module configured such that a light-emitting element and a light-receiving element face each other, so as to obtain a signal associated with the rotation of a motor shaft.
As shown in (A), in order to dispose the code wheel so as to pass through the gap in the photosensor module, first, the code wheel is attached to an end portion of the motor shaft. Subsequently, as shown in (B), the base member on which the photosensor module is mounted is fixedly fitted in position from a direction orthogonal to the motor shaft such that a cut portion thereof is fitted to a mating member. Thus, the mating member is fixed to a motor case in such a manner that the motor shaft extends therethrough; the cut portion of the base member has fixing portions at respective opposite sides thereof, and the mating member and the fixing portions are fixed together in position by use of mounting screws.
The illustrated configuration allows fitting of the base member from a direction orthogonal to the motor shaft for positioning and fixing of the base member. Thus, the photosensor module can be fixedly attached at a predetermined position without damage to the code wheel, which could otherwise result from contact between the photosensor module and the code wheel during attachment.
However, since the mating member must be fixed to the motor case, the motor case must have a special shape such that its end face to which the mating member is attached is, for example, flat. Thus, the mating member cannot be attached to a standard motor. In a standard motor, the end face of the motor case not only is unflat but also has irregularities and components such as a bearing-retaining section and motor terminals. The first prior art technique does not consider attachment to a standard motor and is thus not practical. Additionally, the first prior art technique requires use of screws or the like for ensuring fixation to a motor, resulting in an increase in cost.
FIG. 19 shows a second prior art technique for attaching an optical encoder to a motor. (A) is a perspective view of a small-sized motor to which the optical encoder is attached. (B) shows a sensor unit configured such that a connector and a photosensor module are disposed on a board. After motor terminals are fitted into respective motor terminal insertion slits formed in the illustrated board of the sensor unit, the motor terminals are soldered to the board from the front side of the board (a side of the board opposed to a motor end bell is called the “back side,” and the opposite side is called the “front side”), whereby the board is fixed. This eliminates the need to attach a special mating member to the motor as in the case of the prior art technique shown in FIG. 18 and enables attachment of the optical encoder to a standard motor.
However, since the board of the sensor unit is fixed to the motor merely by soldering to motor terminal portions of the motor, the board is suspended off the motor end bell. FIG. 17 shows this suspended state. If an external force is exerted on an edge portion of the suspended board, the board may bend and be broken. In order to avoid this problem, a glass-epoxy board, which exhibits high strength, must be used. This leads to an increase in cost.
Such an external force may possibly cause bending (plastic deformation) of the motor terminals. If the motor terminals are bent, the positional relationship between the code wheel and the photosensor changes, resulting in a failure to accurately detect a signal. In order to cope with this problem; i.e., in order to make the motor terminals less likely to bend upon imposition of an external force thereon, the distance between a portion (fulcrum) of the motor terminal which is fixed to the motor end bell, and a portion (point of application) of the motor terminal which is attached to the board must be shortened. This narrows the clearance between the board and the motor end bell, resulting in a failure to provide a sufficient working space for soldering the board to the motor terminal portions from the back side of the board. Thus, connection by soldering between the motor terminal portions and a printed wiring portion of the board must be performed at the front side of the board. As in the case of ordinary electronic components, terminals of the photosensor unit and those of the connector extend through respective terminal holes formed in the board and are fixedly soldered to the back side of the board. By contrast, as mentioned above, the motor terminal portions must be fixedly soldered to the front side of the board. Thus, the board must be a double-sided, printed wiring board, whose opposite sides have respective printed wiring portions, and thus becomes expensive. Also, since the motor terminals and the board are connected together by soldering at the front side of the board; i.e., on a side where the photosensor module is present, solder particles and flux may possibly scatter and adhere to the photosensor, causing malfunction of the photosensor.
Since the position (relative position) of the photosensor module in relation to the code wheel is visually determined, positioning of the photosensor is not consistent. Also, parallelism between the motor end bell surface and the board becomes difficult to establish, resulting in inconsistent sensor output.
FIG. 20 is a perspective view showing a third prior art technique for attaching an optical encoder to a motor. (A) and (B) show the same motor as viewed from different directions. The photosensor module and the motor terminals are connected to a printed wiring board and are led to a single region via traces on the board. Input to and output from the photosensor module and the motor terminals are made through the connector. If the printed wiring board assumes such a semicircular shape as shown in FIG. 19, the board will be raised from the motor end bell upon subjection to an external force directed in the direction of the arrow shown in FIG. 20(B). Thus, two leg portions of the U-shaped board are extended to the greatest possible extent so as prevent separation of the board from the motor end surface. However, since the cost of a printed wiring board depends greatly on the number of the printed wiring boards cut out from a single parent board of a predetermined size, such an expansion of surface area leads to an increase in cost.
Also, in the illustrated printed wiring board, a solder side for elements such as the photosensor module is opposite a solder side for the motor terminals. Thus, the printed wiring board must be a double-sided, printed wiring board and thus becomes expensive. By use of surface-mountable elements, the printed wiring board can be a single-sided, printed wiring board. However, the surface-mountable elements must have such heat resistance as to endure reflow soldering and thus are very expensive.
Since the terminals of elements such as the photosensor module cannot be arranged in a region where the board and the motor end bell overlap, the terminals are arranged radially outside of the motor as shown in FIG. 20(B). Accordingly, the board increases in size and thus becomes expensive. Also, since the board radially projects a great distance from the motor, the motor requires a large installation space in the interior of apparatus, such as a printer, in which the motor is installed.
Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2002-357457